a. Coating Processes
The coating, or encapsulation, of pigments with a polymer has frequently been proposed for paint formulations or colorants for molding compositions. The polymer coating can serve several functions including assisting in preventing agglomeration of the particles upon settling or drying and/or facilitating breaking up any agglomerates that do form; facilitating incorporation of the particles into the final coating or molding; binding together pigments of different colors to produce a single desired coating and avoid segmentation of the pigments; and, in the case of reflective pigments such as titanium dioxide, providing a distance between the particles to minimize localized regions of high pigment concentration and thereby maximize hiding power per unit weight of pigment.
These proposals include Newman in U.S. Pat. No. 3,133,893 who discloses pigment particles coated with a polymer which is polymerized in situ by stabilizing pigment particles of less than 1.5 microns, mostly between 0.01 and 0.5 microns, in an aqueous medium in which an emulsion polymerization is carried out to form the polymer coating.
Osmond, et al , in U S. Pat. No. 3,544,500 disclose a complex process for encapsulating solid particles, e.g., pigments. The process involves first adsorbing a polymer on the surface of the particles and providing a stabilizer having an anchor group which becomes associated with the adsorbed polymer on the surface and a pendant hydrophilic component solvated by the aqueous phase and so provides a stabilizing sheath around the particles. The pretreated particles can then be subjected to aqueous emulsion polymerization of a monomer in an aqueous phase to effect the encapsulation. The polymer to be adsorbed on the surface is soluble in an aqueous medium containing the particles and a modification is effected to make the polymer insoluble The patentees disclose at column 3, lines 67 et seq., that the polymer to be adsorbed should contain strong polar groups to promote adsorption on the surface of the particles. At column 2, lines 1, et seq., the patentees state:
"The particles may be dispersed in a solution of the polymer and stabilizer in the liquid but preferably they are dispersed in a solution of the polymer in the liquid, the stabilizer being added after dispersion of the particles. In the second stage of the preparatory treatment the nature of the solution is modified so that the adsorbed polymer is no longer soluble in the liquid continuous phase but the chain-like hydrophilic component of the stabilizing compound remains solvated by it. As a result of the modification of the solution the anchor component of the stabilizer becomes associated with the now insoluble polymer absorbed on the disperse particles whilst the other chain-like component remains solvated by the liquid forming the continuous phase and provides a stabilizing sheath around the particles. The stabilizer may be added before the modification of the solution or at the same time as the modification takes place." PA1 "In general, because of the need to modify the solution containing the stabilizer, the stabilizer will be a non-ionic one providing a steric stabilizing sheath around the particles. Optionally such nonionic stabilizers may be supplemented by ionic types . . . and these ionic types are preferably added after that [preparatory] stage." PA1 Clarke, et al., in U.S. Pat. No. 3,580,880 disclose allegedly stable dispersions and the preparation of same. Particulate solids are said to be dispersed in a liquid with a polymer comprising one chain-like lipophilic pendant component which is capable of being solvated in a lipophilic liquid and at least one nonionic chain-like component which is solvated by a hydrophilic liquid to effect what the patentees refer to as an entropic stabilization (Column 2, lines 15 to 33). The polymer may comprise as the hydrophilic component non-ionic chain-like components with a molecular weight of 350-5000 and the lipophilic component is sufficiently long to provide a barrier 12 Angstroms in thickness around the particles.
The patentees further state at column 3, lines 31, et seq., that:
Schofield in U.S. Pat. No. 4,349,389 describes dispersible inorganic pigment compositions in which the surface of the finely divided inorganic pigment particles is coated with a substance rendering the surface hydrophobic ("hydrophobing agent"), said substance preferably being an amphiphile containing one or more hydrophobic chains of carbon atoms interrupted by O, N or S atoms, and one or more hydrophilic groups such as amino groups, quaternary ammonium groups, carboxylic groups, etc wherein the hydrophilic group or groups anchors the hydrophobic chain to the pigment surface and a layer of a thermoplastic polymer evenly covers and adheres to the hydrophobic chain
Solc in U.S. Pat. No 4,421,660 discloses aqueous dispersions of polymer coated particles, including titanium dioxide pigment particles, produced by first emulsifying polymerizable monomers with a chemical dispersant and/or surfactant and then polymerizing the monomer and coating on the particles with the particles resulting from the polymerization. The patentee discloses the use of high shear conditions, e.g., produced by an ultrasonic probe, to effect a colloidal dispersion of particles. Typically, the dispersion of solid particles is permitted to stand overnight after which the top layer of colloidal dispersed pigment is decanted and used in an emulsion polymerization. The encapsulated particles are particularly intended for applications requiring a colloidal size particulate having magnetic properties. The percent solids contents of the encapsulated particles set forth in the examples were too low without concentration for practical use in the manufacture of water-based paint.
Farrar, et al., in U.S. Pat No. 4,507,422 describe aqueous dispersions of pigments containing as a dispersing agent a water soluble polymer having a polydispersity of below 1.5 (i e., a narrow molecular weight range) and a molecular weight in the range of about 1000 to 5000. The preferred polymers and polymers of acrylic acid (or copolymers) with 2-acrylamido-2-methyl propane
Martin in U.S. Pat. No. 4,608,401 describes finely divided water insoluble particles free of ionic charges and states that the particles are given a generally uniform polymeric encapsulation. The patent states that such materials are obtained by mixing in an aqueous reaction medium a water insoluble monomer polymerizable to form a generally water insoluble polymer free of ionic charges in the presence of a nonionic surface active stabilizing agent, thereafter polymerizing the monomer using a redox polymerization initiating system which is free of ionic groups and does not decompose to release ionic groups in the reaction medium. The patent also states that naturally agglomerated particulate materials are effectively dispersed in situ during polymerization thereby eliminating the need for preliminary grinding and/or dispersion treatments. The patentee emphasizes that the medium be substantially free from anionic or cationic surface active or dispersing agent. The non-ionic stabilizing agent is said to have sufficient hydrophilic-lipophilic power to possess good emulsifying action and have an HLB of at least about 13. The patentee broadly discloses alkylphenyl and derivatives thereof as lipophilic components and ethylene oxide chains as hydrophilic groups, i.e., from about 10 to up to 200 or more --(CH.sub.2 CH.sub.2 O)-- groups. Nonylphenol ethoxylates are the only agents used in the examples.
Solc in U.S. Pat. No. 4,680,200 describes the encapsulation of organic pigment solids in a hydrophobic polymer made by dispersing the monomer in an aqueous colloidal dispersion of the organic particles followed by emulsion polymerization
Gibson, et al., in U.S. Pat. No. 4,413,073 disclose stable aqueous film-forming dispersions. The dispersions are said to be stable to harsh processing such as ball-milling and sand grinding which are sometimes necessary for the incorporation of certain pigments. See column 6, lines 8 to 18. The patentees do not disclose encapsulating particulate solids. Rather, the disclosure pertains to dispersions of particles of film-forming polymer such as alkyd resin in the presence of a stabilizing compound having an HLB value of at least 8 and whose lipophilic portion comprises at least one ethylenic double bond.
Daniel, et al., in European Patent Application 38730 disclose fine iron oxide (Fe.sub.3 O.sub.4) particles (less than 0.01 microns) emulsified in an anionic agent (e.g., C.sub.12 H.sub.25 OSO.sub.3 Na) and non-ionic surfactant, e.g., ethoxylated adduct of nonylphenol having 30 ethylene oxide units.
Many of these proposals recite the use of unsaturated carboxylic acids such as maleic acid, methacrylic acid and acrylic acid as possible monomers for the coating.
In copending U.S. patent applications Ser. Nos. 109,326 and 319,415, filed Oct. 19, 1987, and Mar. 6, 1989, respectively, by Hoy, et al , both entitled Micro Composite Systems and Processes for Making Same, coating processes which are suitable for coating finely divided particles in aqueous dispersions, are disclosed Hoy, et al., use amphiphilic polymers having certain lipophilic and hydrophilic properties to enhance encapsulation efficiencies.
b. Encapsulation Efficiencies
In many instances, the particles which are intended to be coated or encapsulated, e.g., titanium dioxide, are manufactured in a non-agglomerated form. When dried, these particles often agglomerate. Hence, when the particles are to be coated, the agglomerates must be broken up which adds to processing costs and may require additional equipment. Especially for latex paint formulations, particles such as titanium dioxide pigments are available from the manufacturer in stabilized, aqueous dispersions to avoid the need to incur the costs associated with redispersing the particles. Normally, a basic stabilizer is used because of its chemical and pH compatibility with most latex paint formulations.
The use of stabilized aqueous dispersions in coating or encapsulation processes has met with difficulties since these stabilizers tend to reduce the "Encapsulation Efficiency" in the coating process. "Encapsulation Efficiency" is the weight percent of the hydrophobic polymer that coats the particulate solids. The following formula can be used to calculate Encapsulation Efficiency: ##EQU1## Hence, polymer that nucleates to form polymer-only particles reduces the Encapsulation Efficiency
In processes such as disclosed by Solc in U.S. Pat. No 4,421,660, relatively low Encapsulation Efficiencies are typically obtained due to the use of surfactant that readily forms micelles in amounts far in excess of the critical micelle concentration. Hence, the use of stabilized aqueous dispersions of the particles may not result in further undue reductions in Encapsulation Efficiencies. However, in processes which strive toward achieving high Encapsulation Efficiencies, the loss in Encapsulation Efficiencies caused by the stabilizer in the aqueous dispersion is generally unacceptable.
While the stabilizer used in making the stable aqueous dispersion of particles may be changed to one more compatible with the coating or encapsulation processes, this would require reformulations on the part of the particle manufacturer. Moreover, unless the manufacturer maintains separate lines of products, conventional users of stabilized aqueous dispersions may have to reformulate to use an aqueous dispersion made for coating or encapsulation processes. Another approach is to treat the stabilized aqueous dispersion prior to coating such that the stabilizer does not unduly adversely affect the Encapsulation Efficiency. For such a treatment to be practical, it must not only be effective but also be inexpensive and easily accomplished.